In the past, organopolysiloxanes were produced by heating organocyclopolysiloxanes in the presence of an alkali catalyst such as sodium hydroxide, potassium hydroxide, cesium hydroxide, tetraalkylphosphonium hydroxides, tetraalkylammonium hydroxides, and silanolates thereof to effect equilibration reaction. During this reaction, silanol groups are usually formed due to the presence of hydroxyl groups or incidental water and left in the organopolysiloxanes in amount of several hundred to several thousand ppm. Since the residual silanol groups are chemically unstable and can adversely affect the heat resistance of organopolysiloxanes, it is desired to block out the silanol groups.
As is well known in the prior art, silylation was often utilized to modify chemically unstable compounds to be stable. For example, alcoholic hydroxyl groups were silylated using trialkylchlorosilanes or hexaalkyldisilazanes. The silylation using trialkylchlorosilanes, however, required to use dehydrochlorinating agents such as tertiary amines and pyridine for removing the hydrogen chloride by-produced upon completion of the reaction whereas the silylation using hexaalkyldisilazanes required the presence of acid catalysts such as trifluoroacetic acid because the reaction rate is so slow that a long time is taken until completion of the reaction. These silylation methods thus required an extra step of removing the dehydrochlorinating agent or acid catalyst.
Therefore, the organopolysiloxanes resulting from equilibration of organocyclopolysiloxanes in the presence of alkali catalysts could be improved in stability through silylation blocking by using trialkylchlorosilanes or hexaalkyldisilazanes for blocking silanol groups of the organopolysiloxanes, but these methods entailed an extra step of removing the dehydrochlorinating agent or acid catalyst at the end of the blocking step by washing the blocked product with water and heating in vacuum for distillation. This added complexity and cumbersome operation to the overall process and the rate of blocking was far from a satisfactory level. The presence of excess acid catalyst could cause undesirable cracking of the organopolysiloxanes.
Japanese Patent Publication No. 12931/1986 discloses silylation carried out in the copresence of a hexaorganosilazane and a triorganochlorosilane. This method of blocking silanol groups of organopolysiloxanes entailed a step of removing the resultant salt by filtration and required an extremely long time until a high viscosity organopolysiloxane was obtained while the blocking rate was less acceptable.
Since salts were essentially formed during silylation by any of the above-mentioned methods, the end product or organopolysiloxane could be contaminated with ionic impurities unless careful control was made during manufacture. Inclusion of such impurities could be a critical drawback for the organopolysiloxanes which were widely used as potting and adhesive agents in electric and electronic fields, leaving a problem to be cleared prior to their commercial application.
As mentioned above, after organopolysiloxanes were produced through equilibration reaction in the presence of alkali catalysts, no prior art methods were successful in silylation blocking residual silanol groups in a commercially advantageous manner.
Therefore, an object of the present invention is to provide a process capable of preparing a stable organopolysiloxane of quality by silylation blocking residual silanol groups in a simple manner at a high blocking rate.